MOS circuit for lowering forward voltage of diodes

ABSTRACT

A circuit for lowering the effective voltage drop of a semi-conductor diode. The circuit includes a first Metal Oxide Semi-conductor (MOS) device connected in parallel with the diode. A bias voltage which is close to but lower than the threshold voltage is applied to the MOS device. When a forward voltage is applied to the diode, this voltage is added to the source to drain voltage of the MOS device which turns it on. The MOS device then bypasses the diode to effectively overcome the inherent forward voltage drop of the diode. The circuit is advantageously applied to a rectifier circuit to reduce input voltage requirements and improve the efficiency of the rectifier.

FIELD OF THE INVENTION

This invention relates to circuits having semi-conductor diodes and moreparticularly to a system and method of lowering the effective forwardvoltage drop of semi-conductor diodes.

BACKGROUND

Semi-conductor diodes are used extensively in half wave and full waverectifier circuits. It is well known that semi-conductor diodes conductcurrent in a forward direction but substantially no current flows whenthe diode is reverse bias. Further, in the forward direction currentonly flows after the applied voltage exceeds a predetermined voltage andthis voltage is known as the forward voltage drop. The value of theforward voltage drop is dependent on the material from which thesemi-conductor diode is fabricated. The forward voltage drop of asilicon diode, for example, is typically 0.7 volts. It is, of course,known to manufacture diodes by other techniques which may result in alower forward voltage drop. For example, Schottky diodes have a lowforward voltage drop but the process necessary to fabricate Schottkydiodes is not always compatible with the process used in manufacturingthe integrated circuit incorporating the semiconductor diode.

There are several applications wherein the input voltage to the circuitis limited and the forward voltage drop of a standard diode mayadversely affect the efficiency of the circuit such as, for example, arectifier circuit.

In the particular application for which a low forward voltage drop isadvantageous is an RF-tag circuit. RF tag circuits are used tocommunicate various forms of information from and to remote locations.These can be active RF tags which automatically transmit information orsemi-passive RF tags which are activated or interrogated by anotherdevice.

The present invention applies in particular to systems wherein the RFtag is interrogated and in response returns information such aslocation, temperature, humidity, pressure etc. In such systems therectifier in the input circuit rectifies incoming data from an externalsource and extracts from the incoming data the supply voltage (vdd andvss) for the integrated circuit in the RF tag unit.

Obviously, the forward voltage drop can affect adversely is theefficiency of the rectifier circuit. Therefore, there is a need for asystem and method to effectively lower the forward voltage drop ofdiodes used in low voltage environment.

SUMMARY OF THE INVENTION

The present invention provides a simple circuit that can reduce forwardvoltage drop of diodes and provide thereby increased efficiency andoutput voltage of a rectifier circuit.

A current reference controls secondary effects of the circuit. Thecircuit of the present invention is of particular value in integratedcircuit fabrication where standard application processes are employedand Schottky diodes are not available.

The invention is also well suited for low voltage and low powerapplications where the forward voltage of a diode limits the dynamicrange.

Therefore, in accordance with a first aspect of the present inventionthere is provided a circuit for lowering the effective forward voltagedrop-of a semi-conductor diode comprising: a first metal oxidesemi-conductor (MOS) device connected in parallel with thesemi-conductor diode, the MOS device having a set threshold voltage; anda second MOS a device for establishing a bias voltage for the first MOSdevice, the bias voltage being less than the threshold voltage by asmall value.

In accordance with a second aspect of the present invention there isprovided a full wave rectifier circuit comprising first and secondcomplementary diodes, each of the diodes having an associated circuitfor lowering its effective forward voltage drop, the associated circuitcomprising: a first metal oxide semi-conductor (MOS) device connected inparallel with each of the semi-conductor diodes, the MOS device having aset threshold voltage; and a second MOS device for establishing a biasvoltage for the first MOS device, the bias voltage being less than thethreshold voltage by a small value.

In accordance with a further aspect of the present invention there isprovided a method of reducing the effective voltage drop of asemi-conductor diode comprising: connecting a first metal oxidesemi-conductor (MOS) device in parallel with the diode, the first MOSdevice having a predetermined threshold voltage; and generating a biasvoltage for the first MOS device, the bias voltage being less than thethreshold voltage; whereby input voltage applied to the diode is addedto the bias voltage to turn on the MOS device, thereby bypassing thediode.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be described in greater detail with reference tothe attached drawings wherein:

FIG. 1 is a circuit diagram and a prior art rectifier circuit;

FIG. 2 is a circuit diagram of a single diode with bypass circuitry;

FIG. 3 is a circuit diagram of a full wave rectifier circuit using thecircuitry of the present invention; and

FIG. 4 is a circuit diagram of a RF tag circuit using the forwardvoltage drop lowering circuit of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 is a circuit diagram of a known full wave rectifier. A positivevoltage at terminal 12 causes current to flow through diode D1 after theforward voltage drop has been exceeded. Similarly, a positive voltage atterminal 14 causes current to flow through diode D2 again after theforward voltage drop has been exceeded. As indicated previously thisforward voltage drop is dependent on the material used to manufacturethe semi-conductor diodes D1, D2 but for a silicon based diode the valueis 0.7 volts.

FIG. 2 illustrates the basic implementation of the present invention. Inthis implementation MOS device M1 having source, drain and gateterminals is connected as shown. The source and drain connections are oneither side of diode D1. It is, of course, known that in MOS devices theflow of source to drain current is controlled by the gate voltage. Inthe implementation of FIG. 2 the gate voltage is biased close to butless than the inherent threshold voltage Vt of MOS device M1. As isknown the threshold voltage is the gate to source voltage level at whichcurrent flows through the source to drain connections. This bias voltageis derived from MOS device M2 in series with a reference current sourceis as shown.

When an increasing input voltage is applied to diode D1 icy the value ofthis voltage is added to the bias voltage bias which very quicklyovercomes the threshold voltage and MOS device Ml is turned on. The MOSdevice Ml now provides a low resistance path in parallel with diode Dlthereby overcoming the forward voltage drop of the diode.

FIG. 3 illustrates a full wave rectifier circuit comparable to thecircuit of FIG. 1 but now incorporating the bypass circuitry of thepresent invention. In this case complementary metal oxide semi-conductor(CMOS) devices are used in each arm of the rectifier circuit. In theembodiment shown in FIG. 3, MOS devices M1 and M2 provide a bypasscircuit for diode D1 when terminal 12 is provided with a positionvoltage. Similarly MOS devices M3 and M4 provide the bypass circuitryfor diode D2 where a position voltage appears at terminal 14. MOSdevices M1 and M2 and M3 and M4 respectively operate as current mirrorsso that when diodes D1 or D2 are reverse biased the leakage current ofeach diode is set by the reference current and the current mirror ratio.For a low power application, as in an RF-tag application, this currentcan be practically negligible. In the circuit of FIG. 3 the reverseleakage current does not load the external resource circuit as theircurrent is only the difference or mismatch in leakage currents betweenthe two complementary circuits.

In an RF tag application the output voltage using the circuitry of thepresent invention is improved from 1.2 to 1.8 volts. Additionally, thereis a much higher loading capacity due to the lower losses in therectifier.

The diodes D1 and D2 are required in the modified circuit for theinitial time period before vdd is high enough to turn on the currentreference in the integrated circuit.

FIG. 4 is a circuit diagram of an IC implementing an RF tag. As shown onthe input side of the circuit the full wave rectifier incorporates thediode bypass circuitry of the present invention.

While a particular embodiment of this invention has been disclosed andillustrated it will be apparent to one skilled in the art that changescan be made to the circuit without substantially altering the basicconcept of the invention. Such changes, however, will be encompassed inthe full scope of the invention as defined by the appended claims.

What is claimed is:
 1. A circuit for lowering the effective forwardvoltage drop of a semi-conductor diode comprising: a first metal oxidesemi-conductor (MOS) device connected in parallel with saidsemi-conductor diode, said MOS device having a set threshold voltage;and a second MOS device for establishing a bias voltage for said firstMOS device, said bias voltage being less than said threshold voltage bya small value.
 2. The circuit as defined in claim 1 wherein said MOSdevices each have a source, a drain and a gate connections, said sourceand drain of said first MOS device being connected in parallel with saiddiode, and said gate connected to said second MOS device to derive abias voltage.
 3. The circuit as defined in claim 2 wherein said secondMOS device is connected to a reference current source.
 4. A full waverectifier circuit comprising first and second complementary diodes, eachof said diodes having an associated circuit for lowering its effectiveforward voltage drop, said associated circuit comprising: a first metaloxide semi-conductor (MOS) device connected in parallel with each ofsaid semi-conductor diode, said MOS device having a set thresholdvoltage; and a second MOS device for establishing a bias voltage forsaid first MOS device, said bias voltage being less than said thresholdvoltage by a small value.
 5. A full wave rectifier circuit as defined inclaim 4 wherein complementary metal oxide semi-conductor (CMOS) devicesare used in association with said complementary diodes.